1. Field of the Invention
The present invention relates to a method of removing harmful gas from a refuse combustion exhaust gas.
2. Description of the Related Art
A refuse combustion exhaust gas contains harmful gases of nitrogen oxides, sulfur oxides, hydrogen chlorides, and organo-chloric compounds such as dioxine group and the like--all generated from nitrogen, sulfur and chlorine which exist in the refuse. The contents of the first three of these harmful substances are regulated by the law. More specifically, the content of nitrogen oxides should be 80 to 200 ppm or less; that of sulfur oxides should be 30 to 120 ppm or less; and that of hydrogen chlorides should be 300 to 1200 ppm or less. In order to prevent secondary pollution which may occur due to these oxides existing in air, thereby to protect the health of people and preserve their environment, the harmful gases must be removed more efficiently from the refuse combustion exhaust gas. The standards of disposing refuse have actually been made more severe, and gas-removing processes, more complex than the conventional ones, are applied in practice.
Further, although the organo-chloric compound of dioxine is not regulated in our country at present in Japan, for example, the harmfulness thereof becomes a serious issue. However, the real condition is that no counterplan is carried out to cope with such issue.
There are two methods generally employed of removing nitrogen oxides, sulfur oxides, and hydrogen oxides (not organo-chloric compounds such as dioxine and the like). The first is a wet process, and the second is a semi-dry process.
FIG. 3 shows the wet process.
(1) The high temperature exhaust gas having temperature of 750.degree. to 950.degree. C. generated from an incinerator is cooled down to 250.degree. to 350.degree. C. in a boiler 2. PA1 (2) Dust in the exhaust gas is removed in a dust collector 3. PA1 (3) NH.sub.3 gas is blown and the resulting gas is led into a catalyst tower 4 for reducing nitrogen oxides, and the nitrogen oxides are removed by reducing in accordance with a reaction formula represented by following. EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O PA1 (4) The exhaust gas is washed in a tower 5 for removing hydrogen chloride and sulfur oxides using a large amount of water. At this time, in order to maintain a neutral washing, an alkaline material represented by a sodium hydroxide is added. The resulting reaction at this time is as follows. EQU HCl+NaOH.fwdarw.NaC+H.sub.2 O EQU SO.sub.2 +2NaOH+1/202.fwdarw.Na.sub.2 SO.sub.4 +H.sub.2 O PA1 (5) Since the washed gas from which the harmful contents are removed has a temperature lower than 100.degree. C., the gas is heated to 120.degree. to 180.degree. C. by a reheater 6 and is diffused to the air from a funnel 7 in order to a) prevent a dew point corrosion of duct and funnel, b) prevent water from dropping from top portion of funnel, c) improve a diffusion efficiency, and d) prevent occurrence of steam white smoke from the funnel, and so on. PA1 (1) The high temperature exhaust gas having temperature of 750.degree. to 950.degree. C. generated from the incinerator 1 is cooled down to 250.degree. to 350.degree. C. in the boiler 2. PA1 (2) Ca(OH).sub.2 slurry is blown from the top portion of a tower 8 for removing hydrogen chloride and sulfur oxides as the alkaline material, wherein such slurry is atomized finely to diffuse within the tower, thereby removing hydrogen chloride and sulfur oxides by neutralizing in accordance with reaction formulas represented by the following. EQU 2HCl+2Ca(OH.sub.2).fwdarw.CaCl.sub.2 .multidot.Ca(OH).sub.2 .multidot.H.sub.2 O+H.sub.2 O SO.sub.2 +Ca(OH).sub.2 +1/20.sub.2 .fwdarw.CaSO.sub.4 +2H.sub.2 O PA1 (3) Salt which is a solid state product produced by the above mentioned formulas is removed and recovered by the dust collector 3. PA1 (4) NH.sub.3 gas is blown, and the resulting gas is led into the catalyst tower 4 for reducing nitrogen oxides to reduce and remove the nitrogen oxides in accordance with the following formula. EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4H.sub.2 +6H.sub.2 O PA1 (5) The disposed gas is diffused to the air through the funnel 7. PA1 (1) Since the gas temperature has to be heated to more than about 250.degree. C. in order to prevent bindings between catalyst layers in the catalyst tower 4 for reducing the nitrogen oxides, a boiler recovery heat quantity has to be restricted. The blindings are caused by producing an acid ammonium sulfate (NH.sub.4 HSO.sub.4), which leads to an unstable operation. PA1 (2) Since the catalyst to be used is very expensive and it has to change or reproduce periodically, the maintenance becomes a very complicated one. PA1 (3) In order to have the structure for preventing blindings, the volume for the device of catalyst layer becomes large. PA1 (4) Since the gas temperature is high about (250.degree. C.), the volume for the device of the dust collector 3 becomes large. PA1 (5) The excess NH.sub.3 and ammonium chloride generated by hydrogen chloride are often noted as white smoke, thereby causing complaints from the residents who live in and around the incinerator. PA1 (6) It needs to deal with NH.sub.3 which is a poisonous substance. PA1 (7) A large amount of water of 0.7 m.sup.3 per 1 t of refuse is needed as a makeup water for washing gas in the tower 5 for removing hydrogen chloride and sulfur oxides. PA1 (8) Since the washing contains three to five times salting concentration as sea water, it is not possible to make a wastewater treatment directly. Therefore, about 0.6 m.sup.3 dilution water is needed every 1 t of refuse. PA1 (9) A complicated equipment for wastewater treatment is needed. PA1 (10) In the reheater 6 for solving the above mentioned problems caused by the fact that the gas temperature after washing is low, a large amount of heat quantity (about 250,000 Kcal per 1 t of refuse) is needed.
Salt produced by the above mentioned formulas is extracted with one portion of the washing and are disposed by processes such as coagulating sedimentation, filtration, a biological process and so on after dilution with fresh water.
FIG. 4 shows the semi-dry process.
Apart from the exhaust gas from the incinerator, a method is shown in Japan Laid-Open Patent No. 17471173 for activating by irradiating an activating rays against the harmful gas of sulfur dioxide and nitrogen oxides and so on in the combustion exhaust gas from a thermal power plant in which a fuel oil and coal are used as fuel, to realize decomposition or to render the gas harmless.
U.S. Pat. No, 4,372,832 shows a method for disposing exhaust gas in case of where a fuel including a large amount of sulfur is used. According to this method, the exhaust gas is washed by a spray drying method and the temperature of the resulting gas is lowered to increase hydroscopic moisture thereof. Then, the resulting exhaust gas is disposed with an electron beam radial rays.
U.S. Pat. No. 4,524,142 also shows the method for disposing exhaust gas in case of where the fuel including a large amount of sulfur is used. According to this method, first lime stone is supplied into the boiler. Then, the exhaust gas from the boiler is humidified and the temperature thereof is lowered at the same time. The resulting gas is disposed with the electron beam radial rays.
There are common problems in the wet process as shown in FIG. 3 and the semi-dry process as shown in FIG. 4 as followings.
Further, as regards the problem inherent in the wet process as shown in FIG. 3, there are problems as follows.
Japanese Laid-Open Patent Publication No. 17471/73 discloses the invention to remove the harmful gas contents in the combustion gas of fuel oil and cool and so on, for example, sulfur dioxide and nitrogen oxides, and it is questionable whether the above invention function effectively against the refuse combustion exhaust gas including large amount of hydrogen chloride as high as ten times the harmful gas disclosed therein.